1. Field of the Invention
The present invention relates to radiation systems.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Typically, lithography systems use lasers as radiation sources to produce an illumination beam. The lasers typically are comprised of an oscillator, while high power lasers are comprised of a master oscillator, which generates a beam of radiation, and a power amplifier, which amplifies the beam. The amplified beam is output as the laser beam. These lasers have a random variation in pulse energy, and also in other parameters of the beam, such as position, pointing, size, and divergence.
For mask based scanners, the mask is imaged on the wafer using several tens of laser pulses. During wafer exposure, control algorithms maintain the pulse energy, beam position, and beam pointing at an illuminator entrance (averaged over the amount of pulses in the exposure) within the specified limits.
However, for a maskless system, the desired scenario (because of throughput reasons) is that the pattern or patterning device is imaged on the substrate in a single pulse. Lasers have an energy variation for a single pulse of up to about +/−10%, which is much too large for single pulse exposure. Thus, it is no longer possible to rely on a control algorithms that compensate for deviations of previous pulses and the averaging effect over the amount of pulses in the exposure because they are no longer applicable. In addition to the pulse energy stability, the stability of the laser beam shape (e.g., position, pointing, size, and divergence) needs to be improved for a maskless system.
Reduction in pulse energy variation can be performed through trimming of the energy of an individual pulse using a fast detector and a fast optical shutter (e.g., both having nano-second response time) in combination with an optical delay line. For example, this is done in U.S. Pat. No. 5,852,621, which is incorporated by reference herein in its entirety. The fast optical shutter can be a Pockels cell that uses an electro-optic material, e.g., an electro-optical modulator. The Pockels cell has been made from materials such as Potassium Di-hydrogen Phosphate (KDP) and Lithium Triborate (LBO). The problem is that these materials are not very effective at the smaller and smaller wavelengths being used today in lithography systems to form smaller and smaller elements. For example, at wavelengths of 193 nm and below these materials exhibit poor transmission and/or have a short lifetime at 193 nm.
Therefore, what is needed is a system and method that produce a radiation beam having better pulse-to-pulse uniformity for small wavelengths.